1. Field of the Invention
The present invention generally relates to a polarizing beam-splitter, and especially relates to a high performance multilayer thin-film broad-band polarizing beam-splitter for obtaining two orthogonal linearly polarized beams with a high extinction ratio from an unpolarized beam of broad spectral range. The absorption loss of the polarizing beam-splitter, made of multilayer thin-film coatings, is negligible because the coating materials used in the thin-film stack are virtually lossless dielectric materials.
2. Description of the Related Art
Thin-film multilayer polarizing beam-splitters are commonly used in contemporary optoelectronic systems. However, most have tradeoffs between high optical throughput and high extinction performance unless they are operated at single wavelength or in a narrow spectral band. Thus, the demand for broad-band polarizing beam-splitters increases as applications across the whole visible range have become increasingly important.
A first related prior art had been disclosured by R. Messner, "Die theoretischen grundlagen optischer interferenzpolarisatoren", Feinwerktechnik 57, 142-147(1953). The first prior art used 15 plates, each with quarter-wave films coated on both sides, parallel-stacked with air gaps to provide approximately 90% polarized light transmission at an oblique angle of incidence. They are called pile-of-plates polarizers. The first prior art had drawbacks of complicated stacking procedure, larger walk-off of the transmitted light, narrow operating bandwidth, and especially widely-spreaded reflected beams that were not orthogonal to the transmitted beam. The last drawback greatly limited the application of such devices as polarizing beam-splitters.
A second related prior art had been disclosed by H. Schroder, "Die erzeugung von lineraploarisierttem licht durch dunne dielektrische schichten," Optik 3, 499-503(1948). The second prior art was to operate the pile-of-plates polarizers at a particular angle such that only light with polarization perpendicular to the plane of incidence would be reflected. Though this improved the extinction ratio and resulted in far fewer plates being necessary for equal performance, it still suffered from the same disadvantage of not being able to allow such polarizers to function as polarizing beam-splitters.
A third prior art replaced the multiple plates with thin layers of low refractive index material to enhance the interference effect. The multilayers of high and low-refractive index materials were on a plate or embedded between materials having another suitable refractive index. The bandwidth of these polarizers was much smaller than that of pile-of-plates polarizers. Choice of the refractive indices and angle of incidence had strong influence on the bandwidth and the extinction ratio of thin-film multilayer polarizers. Since broad-band thin-film polarizers are extremely difficult to make, the extinction ratio is traditionally improved by passing the incident radiation through multiple stages of the polarizing process, such as pile-of-plates polarizers, with the expense of optical throughput. However the reflected beams of such polarizers can hardly be combined into a single light beam with orthogonal polarization from the transmitted beam. Therefore, it is not satisfactory to use the above prior art as a polarizing beam-splitter.